The present invention generally relates to a system for evaluating and tuning a radiation generator. In particular, the present invention relates to an apparatus intended for (a) performance evaluation of an x-ray generator, (b) evaluation of associated imaging system signals for servicing, (c) optimization of radiographic parameters for image contrast, patient entrance exposure and x-ray tube loading, and (d) on line feed back of performance parameters to x-ray generator control for improving the radiation output characteristics. This device, which is capable of handling several radiation induced and non-radiation based signal inputs, combined with novel data sampling and processing methods, provides a self-consistent method of measurement for every single x-ray exposure.
X-ray imaging is a popular clinical diagnostic imaging modality. X-rays cause damage to tissues due to their ionizing power. Image quality is fundamental to diagnosis while minimal x-ray exposure reduces patient radiation risk. In order to effect these twin goals, government bodies and professional organizations have enacted performance standards for x-ray imaging systems. Performance of x-ray systems is determined by measuring radiation. If a system is found to be non-compliant with the expected standards, corrective action is taken and servicing of the equipment is performed. Service personnel use test devices to measure electrical signals and identify problems for correction. An apparatus, which can provide both types of measurements, would help quick service and recalibration. Radiation based measurements, [ref. 12,9,7,6,5,4,3,1] currently used, include kVp (kilovolts peak), mA (milliamperes), HVL (half value layer), radiation exposure [ref. 8,2,1], exposure time, and radiation waveform [ref. 12].
X-ray energy spectrum is continuous, modeled by bremsstruhlung theory, with the highest energy of the spectrum [ref. 11] determined by the peak applied potential kVp. For the same kVp, different types of generators such as single phase, three phase six pulse, three phase twelve pulse, single phase medium frequency (variable), or high frequency inverters would produce different energy distributions. The mA determines the intensity or number of x-rays of an exposure without changing the energy distribution of x-rays. Since kVp can not describe the energy of radiation due to the spectral distribution, measurement of Half Value Layer (HVL) representing the effective energy with respect to attenuation by aluminum is necessary. This is usually obtained using data in several separate exposures of radiation output placing different thickness of aluminum filters in the path of radiation. This measurement procedure requires the x-ray generator is working under reproducible conditions. This assumption is questionable, particularly in the context of quality control testing. If we have determined the distribution of applied voltage and attenuation of radiation at each voltage for several thickness of filters, for the same radiation exposure simultaneously, then we can compute HVL consistently. In order to accomplish the above measurements, several novel design concepts have to be developed and implemented.
From a servicing point of view, rise time of x-ray exposure, pulse overshoot, and any high voltage breakdown due to x-ray tube problems require sample times in the range of 10-50 microseconds. Devices with analog electronics having a bandwidth of 1 kHz and 10 KHz analog-to-digital converter system can not detect these problems.
For self-consistent x-ray measurements, radiation parameters [ref. 10] such as kVp. HVL, mA, exposure time, exposure or kerma should be evaluated at the same time for an exposure. Corresponding waveforms should be processed for the same exposure as well. If this can be accomplished, this device would reduce number of exposures for evaluation, identify system performance with minimal uncertainty and self-consistency.
Non-invasive methods of estimating kVp from radiation measurements have proven to be useful. Currently popular method of kVp evaluation is based on radiation measurements using differential methods. The kVp measurement, ideally, should be performed for the whole exposure. Several designs have set a practical limit on the inclusion of exposure data for 100 milliseconds to 300 milliseconds. Actual exposures, as per generator specifications can be as long as 10 seconds. Exposure stability problems could appear in a long exposure, as x-ray tubes become gassy. Thus, exposure time limit for kVp measurement leaves critical problem areas unattended.
Among the inputs to the performance evaluation apparatus, two classes of signals are involved. Signals from x-ray assembly are fast, following the generator frequency. Signals from ionization chamber, mA meter, photo-timer output are integrated signals with less than 1 KHz bandwidth. Signals from circuits for exposure start, exposure terminate, rotor ready, filament ready etc. are, perhaps, pulses with several seconds of delay between their occurrence.
Accordingly, the present invention offers the flexibility for measuring any combination of signals, thus advancing measurement procedures to technological limits.
This invention has accomplished to overcome the design limitations by devising a suitable component architecture and implementation. Measurement limitations have been removed by designing a multiple sensor inputs assembly and an automatic scan method to sample high frequency and low frequency signals from radiation sensors and electrical interfaces in the same exposure. Suitable computational procedures have been developed to arrive at accurate performance parameters and waveforms of the x-ray generator. Thus, this invention has succeeded in achieving self-consistent performance parameters of x-ray system. In addition to performance evaluation, this apparatus is useful for optimization of patient entrance skin exposure and image contrast. This apparatus can be extended to feed performance information back to generator control for on line adjustments for improved performance levels for accurate x-ray imaging applications.
In general, a system is provided that includes a multiple sensor assembly, a filter assembly, and a processor assembly. The multiple sensor assembly has a plurality of radiation sensors arranged to receive a radiation signal from a radiation generator. Each radiation sensor has a sensor output for providing a radiation sensor signal to the processor assembly. The filter assembly has a filter panel for at least one of the radiation sensors with each filter panel having an associated radiation sensor and being operably interposed between the radiation generator and its associated radiation sensor. The processor assembly is operably connected to the multiple sensor assembly to communicate with the multiple sensor assembly in order to receive the radiation sensor signals for evaluating the performance of the radiation generator.
In one embodiment, the invention includes (1) a multi-sensor assembly, (2) personal general purpose computer controlled electronics for signal conditioning and optimization, (3) a personal computer interface card that includes a programmable gain amplifier, multiplexer, analog-to-digital converter and digital input-output interface, (4) a personal computer with storage, (5) an application software or firmware for x-ray system performance evaluation and (6) personal computer compatible input and output devices. The multi-sensor assembly includes several x-ray sensitive sensors, which are substantially more efficient [ref. 18,14,13,1] for x-rays than simple silicon photodiodes. This is accomplished either by optically coupling silicon photodiodes with x-rays-to-light converting screens or materials used in radiography and fluoroscopy or using large area photo-conductive devices. The sensor devices are operated with bias and with device output optimized by load resistors. The computer controls the operating conditions and load on the sensors, through software and analog switches, depending on the signal and prior knowledge of x-ray system test conditions available from an integrated database. The system has the capability to determine optimal operating conditions of signal level, noise, and bandwidth for each exposure, set the correct conditions, and collect signal data under optimal conditions. Each sensor is independently optimized, amplified, and digitized without any specific electronic configuration determining the application of a particular sensor for the purpose of a parameter measurement such as kVp, kerma or exposure, time etc.
Each sensor signal is amplified with a fixed gain. At the time of digitization, the signal is amplified by a programmable gain amplifier (PGIA) and digitized to, say, 12-bit accuracy. The computer controls the gain of PGIA for the sensor signal, through software on the current and prior knowledge of x-ray system test conditions available from the integrated database. The system has the capability to determine optimal operating conditions for each x-ray exposure, set the correct conditions and digitize signal data under optimal signal conditions.
Sensor load, amplifier and PGIA cause an offset voltage signal. This offset requires cancellation before digitization so that actual radiation induced signal can be measured accurately without sacrificing the dynamic range of the analog-to-digital converter (ADC). Offset cancellation is accomplished by using differential inputs to PGIA with signal input and input from a digital-to-analog converter (DAC). When there is no radiation input, sensor signal is digitized and using this value the DAC is set with digital values to produce output voltage close to the offset. This process is continued iteratively to achieve a minimal offset of the system including ADC by software control from the computer. A FilterPak including radiation filter elements is placed in a fixed position assembly intercepting x-rays reaching sensors. The choice of materials and the thickness of the filter element is performed depending on (1) x-ray application such as general radiography, dental radiography, mammography etc., (2) type of x-ray tube such as tungsten anode, molybdenum anode etc., and (3) type and thickness of added filtration. The number of filter elements and sensors is decided based on the x-ray application and accuracy required for self-consistent determination of x-ray performance parameters.
Inputs from external electrical signals such as kV, mA, photo-timer, lightmeter, external radiation sensors etc., can be fed to PGIA and digitized in real time along with the radiation sensors of the multi-sensor assembly. Service engineers require many of these external signals for trouble-shooting and servicing. Thus the present invention combines the functions of traditional x-ray quality assurance (QA) devices and oscilloscope and excels in performance by acquiring these waveforms in real-time. In one embodiment of the invention, an integrated database is used; it can contain stored information of the following: (1) Calibration system details and performance characteristics, (2) Calibration curve parameter fits, (3) Sensor and electronic signal control and optimization information, (4) Information related to data acquisition signal order, sampling interval between successive sensor signals, sampling period, and data acquisition duration, (5) External signal conditioning and control information, (6) Information related to performance parameter accuracy and permissible deviations pertaining to regulatory requirements, (7) Information of measured performance parameters and waveforms to produce performance trends of parameters, (8) Information on diagnostics of performance or trouble-shooting of tested x-ray system, (9) Information on solutions or tips to overcome the performance problems, (10) Information on optimizing the entrance air kerma or patient dose for the x-ray system under test based on the measured performance parameters and theoretical parameters simulating x-ray system characteristics, and (11) Information on the corrections of performance parameters due to variations of x-ray system characteristics between calibration system and x-ray system under test. This database information is used automatically for the operation of this apparatus.
This invention accomplishes several functions based on x-ray system tests: (1) Regulatory compliance and QA checks, (2) System performance diagnosis and guidance for solutions, (3) System optimization in terms of patient entrance skin exposure or air kerma, (4) Optimization of x-ray exposure techniques, and (5) Optimization of image contrast. This apparatus is also a novel tool for service engineers for interactive calibration and adjustments of x-ray systems, such as general radiography, mammography, fluoroscopy, angiography, cardiography, and single plane and biplane special procedure systems.
An integrated spreadsheet permits creation of standard and custom styles or formats for reports, which may include text, tables, and color graphs. Processed results are automatically presented as reports in spreadsheet style. User can choose the graphs of interest from a dropdown list and resize as required for the report. Reports are displayed on the screen and are available for hardcopy output through any PC-media.
The present invention has implemented a system of self-consistent measurement of x-ray system performancexe2x80x94automatically measuring kVp, HVL, kerma, exposure time, and all related waveform parameters such as rise time, ripple, fall time, spikes, break-downs etc., in a single x-ray exposure. This is a fundamental part of this invention. This method overcomes the uncertainties in determining performance parameters using several separate x-ray exposures. Multi-sensor arrangement in conjunction with suitable FilterPak helps acquiring self-consistent radiation data for evaluation of parameters automatically. Using appropriate FilterPak, the self-consistent evaluation method is applicable for any x-ray system used for general radiography, mammography, dental radiography, fluoroscopy, angiography, cardiography, and single plane and biplane, special procedure systems etc.
In addition to regulatory checks of performance standards of individual parameters, this apparatus analyzes systematically the performance parameters based on complete test. Behavior of x-ray tube output, HVL, relative mA, pulse frequencies in radiation waveforms, ripple, and rise and fall time values are considered with respect to, for example, with respect to kVp are considered for any substantial change to identify any corrective or preventive steps. This would have an appreciable impact in early diagnosis and suitable preventive maintenance avoiding surprise system failures and saving costs.
By choosing suitable material and thickness mimicking tissues for the FilterPak, image contrast performance of x-ray system can be studied for exposure without using film. This device permits image contrast evaluation using sensor signals quickly and accurately for range of operation of the system. Theoretical guidance using measured data is offered for optimizing contrast by selection of suitable kVp, and added filtration material and thickness. At the same time, patient entrance skin exposure or air kerma is also reduced by choice of appropriate added filtration material and thickness. This approach to optimization of contrast, technique (kVp) and patient exposure (dose) using FilterPak is applicable to any x-ray system used for general radiography, mammography, dental radiography, fluoroscopy, angiography, cardiography, and single plane and biplane, special procedure systems etc. This optimization capability is also a unique aspect of the invention. In combination with system performance evaluation capabilities, this apparatus provides a simple and practical method to improve the imaging performance of the x-ray system while minimizing patient dose.
The present invention is capable of self-consistent performance evaluation for radiographic x-ray exposures (radiation pulse) and fluoroscopic exposures (continuous radiation). For fluoroscopy, determination of kVp, exposure (dose) rate, HVL etc. are performed simultaneously. In addition, stability of these parameters over a period of several minutes can be determined as well. For x-ray systems, which use film camera (70 mm, 105 mm, cine camera etc.) or digital camera this apparatus measures self-consistent performance parameters for each exposure pulse of the complete exposure including several pulses. Stability, mean and deviations of self-consistent parameters of exposure pulses over the whole run are also evaluated. Waveform analysis is also performed for the whole exposure run with the same data.
For correct operation of an x-ray system, a specific real time sequence of several electrical signals from generator control and generator system component interfaces are required by design. Acquiring these electrical control and interface signals along with radiation signals would help quick inspection, calibration, and trouble-shooting of the x-ray system. The present invention offers a method of assessment of real time signal sequences of electrical and radiation signals, and evaluation of time intervals, delays and self-consistent radiation based parameters for every x-ray exposure sequence.
The present apparatus provides several types of softcopy and hardcopy reports based on measurements, and theoretical simulated parameters: (1) Single exposure report for each x-ray exposure, (2) Summary report of a complete test, (3) Trends of parameters over several past tests, (4) System Diagnostics report, and (5) System solutions report. Each report may include waveforms, as selected by user. Reports are customizable as well.